Electrically conductive elastomer

ABSTRACT

An electrically conductive polyurethane elastomer obtained by the reaction of a polytetramethylene ether glycol having the formula HO[(CH 2 ) 4  O] x  H where x is from 8 to 41 and from about 20 to about 95 parts by weight per 100 parts by weight of said glycol of at least one diisocyanate, a sufficient amount of chain extender and cross linking agents to provide a crosslinked elastomer and an amount of an asymmetric ionic quarternary ammonium salt having the formula: ##STR1## where R 1 , R 2 , R 3 , R 4  and R 5  are C n  H 2n+1  and 1≦n≦25 sufficient to provide a D.C. volume resistivity of from about 10 7  to about 10 11  ohm cm. In a preferred embodiment the asymmetric ionic quarternary ammonium salt is hexadecyl ethyl dimethyl ammonium ethyl sulfate.

CROSS REFERENCE TO RELATED APPLICATION

Attention is directed to U.S. application Ser. No. 08/037,145 entitled"Electrically Conductive Polyurethane Elastomer" in the name ofSchlueter, et al., filed concurrently herewith.

BACKGROUND OF THE INVENTION

The present invention relates to an electrically conductive polyurethaneelastomer and in particular to asymmetric ionic quarternary ammoniumsalts to extend the electrical life of polyurethane elastomers. It hasparticular application in electrostatographic printing apparatus and inparticular as a biased transfer member in transferring toner from anelectrostatographic imaging surface to a receiving surface such as asheet of paper.

In the process of electrophotographic printing, a photoconductivesurface is charged to a substantially uniform potential. Thephotoconductive surface is image wise exposed to record an electrostaticlatent image corresponding to the informational areas of an originaldocument being reproduced. This records an electrostatic latent image onthe photoconductive surface corresponding to the informational areascontained within the original document. Thereafter, a developer materialis transported into contact with the electrostatic latent image. Tonerparticles are attracted from the carrier granules of the developermaterial onto the latent image. The resultant toner powder image is thentransferred from the photoconductive surface to a sheet of supportmaterial and permanently affixed thereto.

This process is well known and useful for light lens copying from anoriginal and in printing applications from electronically generated orstored originals.

In a reproduction process of the type as described above, it is commonpractice today to use a biased transfer member to transfer the developedimage from the photoconductor to the final support material such as asheet of paper. Typically, these biased transfer members take the formof a roll and are comprised of a polyester based polyurethane with anadditive to control resistivity such as tetraheptyl ammonium bromide.While capable of performing satisfactorily as a bias transfer member, ina transfer system wherein only about 25 microamps are applied to thetransfer member difficulties are experienced in other systems,particularly, in multicolor imaging systems where more than one tonerand a greater pile height of toner on the photoconductor must betransferred to the copy sheet. These systems require additional currentup to 75 microamps. While the polyester based polyurethanes containing atetraheptyl ammonium bromide operate satisfactorily at low current loads(25 microamps) having an electrical life of up to 1,500,000 prints atthese increased current loads (75 microamps) we have found that theelectrical life is shortened to approximately 500,000 prints. This isbelieved to be due to the depletion of the conductive additive in thepolyurethane, in that the ionic components want to separate and overtime the conductive additive is depleted, the resistance of the transfermember increases and under constant current the voltage on the powersupply maximizes leading to a transfer system failure. This is believedto be due to the fact that the present systems are very low crosslinkedsystems and therefore the additives have high mobility and are veryeasily transported through the elastomer network, resulting in thediffusion of the charged components and accordingly reduction inconductivity of the transfer member. These prior art systems aredescribed in U.S. Pat. No. 3,959,574 to Seanor et al. and U.S. Pat. No.3,959,573 to Eddy et al.

Furthermore, attempts to use a new polyether based polyurethaneexhibiting high wear resistance and low compression set as described incommonly assigned copending application Ser. No. 07/767,442 entitled"High Wear Resistance Low Compression Set Polyurethane" filed Sept. 30,1991, in the name of Schlueter et al. as a biasable member with thetetraheptyl ammonium bromide additive met with failure in that thetetraheptyl ammonium bromide poisoned the catalyst so that the degree ofcross linking was inadequate.

SUMMARY OF THE INVENTION

In accordance with a principle aspect of the present invention anelectrically conductive polyurethane elastomer having an extendedelectrical life is provided. More specifically, a polyether basedpolyurethane contains an asymmetric ionic quarternary ammonium saltwhich will extend it's useful electrical life. It is believed that byproviding a very asymmetric additive as well as a highly cross linkedelastomer that the mobility of the additive through the polymer networkof the elastomer is reduced and accordingly the useful life of theconductive additive extended. The use of a very asymmetric conductiveadditive is in sharp contrast to the symmetrical tetraheptyl ammoniumbromide used in the above referenced prior art systems.

In a further aspect of the present invention an electrically conductivepolyurethane elastomer is obtained by the reaction of apolytetramethylene ether glycol having the formula HO[(CH₂)₄ O]_(x) Hwhere x is from 8 to 41 and from about 20 to about 95 parts by weightper 100 parts by weight of the glycol of at least one diisocyanate, asufficient amount of chain extenders and cross linking agents to providea crosslinked elastomer and an amount of an asymmetric ionic quarternaryammonium salt having the formula: ##STR2## where R₁, R₂, R₃, R₄ and R₅are C_(n) H_(2n+1) and 1≦n≦25 to provide a D.C. volume resistivity offrom about 10⁷ to about 10¹¹ ohm cm. In a particularly preferredembodiment the quarternary ammonium salt is hexadecyl ethyl dimethylammonium ethyl sulfate.

In a further aspect of the present invention the electrically conductivepolyurethane elastomer is used as a coating on a conductive substrate toform a member for electrically cooperating with an imaging surface toattract charged toner particles from the support surface toward themember which has a uniform bias potential thereon and is capable oftransmitting a bias potential from the substrate to the outer peripheryof the coating.

In a further aspect of the present invention the diisocyanate isselected from the group consisting of methylene diisocyanates,diphenylmethane diisocyanates, toluene diisocyanates, naphthalenediisocyanates and blends thereof.

In a further aspect of the present invention the chain extenders andcross linking agents comprise a mixture of from about 90 to 60% byweight of a diol having the formula HO(R₆) OH where R₆ is a straight orbranched chain alkyl group having from 2 to 12 carbon atoms and fromabout 10 to 40% of a triol having the formula: R'--C--[(OH)_(a) (CH₂OH)_(b) ] where R' is H, CH₃ or C₂ H₅, a is 0 or 1, b is 2 or 3 anda+b=3.

In a further aspect of the present invention the quarternary ammoniumsalt is present in an amount of from about 0.5 parts to 8 parts byweight of the total composition.

In a further aspect of the present invention the elastomer may beprepared by adding all the recited reactant constituents and chainextenders and cross linking agents to a reaction vessel at the same timeor a prepolymer may be prepared of the polytetramethylene ether glycoland the diisocyanate followed by the addition of both the chainextenders and cross linking agents to the prepolymer.

In a further aspect of the present invention the total weight of chainextenders and cross linking agents is from about 4 to about 18 parts per100 parts of polytetramethylene ether glycol.

In a further aspect of the present invention the elastomer is formedfrom a mixture of a chain extender and a cross linking agent of about75% by weight of 1,4 butanediol and 25% by weight trimethylolpropane.

In a further aspect of the present invention the elastomer is formedfrom a mixture of a chain extender and a cross linking agent of fromabout 60% by weight 1,4 butanediol and 40% by weight trimethylolpropane.

In a further aspect of the present invention x is from 39 to 41.

In a further aspect of the present invention the diisocyanate is presentin an amount from about 22 to 26 parts by weight per 100 parts by weightof said glycol.

In a further aspect of the present invention the quarternary ammoniumsalt is initially combined with the chain extenders and cross linkingagents and subsequently added to the prepolymer.

For a better understanding as well as other objects and further featuresthereof, reference is made to the following drawings and descriptions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view in partial section showing the constructionof a bias transfer roll having an electrically conductive polyurethaneelastomer containing an asymmetric ionic quarternary ammonium saltadditive coated upon a conductive substrate.

FIG. 2 is a perspective view in partial section showing the constructionof a bias transfer roll having an elastomeric resilient blanket orrelaxable layer intermediate, the conductive substrate and theelectrically conductive polyurethane elastomer with an asymmetric ionicquarternary ammonium salt additive coated thereon.

DESCRIPTION OF PREFERRED EMBODIMENT

The electrically conductive polyurethane elastomer, and in particular,the biasable transfer members made therefrom have application in avariety of electrostatographic devices. In particular, the biasabletransfer member, such as a roll, is capable of electrically cooperatingwith an imaging surface to attract charged toner particles from theimaging support surface toward the member since the coating is capableof transmitting a bias potential from the substrate to the outerperiphery to thereby attract charged toner particles. By selecting anasymmetric conductive additive the mobility and diffusion of theadditive through the elastomer is dramatically reduced so that theresistivity stays relatively stable. As a result a constant current andmuch slower rate of changing of the resistivity of the transfer memberis achieved in general and may provide an electrical life extension ofup to six times.

Referring now specifically to FIG. 1, there is shown a cut-away view ofa transfer member clearly illustrating the internal constructionthereof. The transfer member is in the form of a roll and is basicallyformed upon a rigid hollow cylinder 2 that is fabricated of a conductivemetal, such as aluminum, copper or the like, capable of readilyresponding to a biasing potential placed thereon. Over core 2 is placeda coating 4 which is an electrically conductive polyurethane elastomerwith an additive according to the present invention to render theelastomer conductive and to extend the rolls' useful life. The outercoating 4 which is formed of the resilient elastomeric material is fromabout 0.001 inch to about 0.250 inch in thickness having a hardnessbetween about 60 Shore A durometer to about 95 Shore A and preferablyabout 85-95 when the thickness is less than 0.100 inch and 65 to 75Shore A when the thickness is greater than 0.100 inch. The coating 4minimizes ionization of the atmosphere in and about the contact regionof the bias transfer member with the photoconductor. It is preferredthat the resilient elastomeric polyurethane have a D.C. volumeresistivity of between about 10⁷ and 5.0×10¹¹ ohm cm which is reached orcontrolled by adding the additive to the polyurethane. In accordancewith the present invention, the coating of the conductive substrate mustbe formulated of at least one layer of an elastomeric polyurethanehaving as an additive, a compound capable of altering the resistivity towithin the preferred resistivity range. By coating the biasable transfermember (roll) with the conductive elastomer, according to the presentinvention, the resistivity of the biasable transfer roll is controlledand the sensitivity of the resistivity of the biasable transfer roll isalso controlled in relationship to changes in relative humidity.

Referring now to FIG. 2, wherein there is shown a cutaway view of abiasable transfer roll and a clear illustration of the internalconstruction thereon, there is shown a rigid hollow cylinder 2 that isfabricated of a conductive metal, as in FIG. 1, capable of readilyresponding to a biasing potential placed thereon. Over the rigid hollowcylinder 2 is placed a relatively thick resilient intermediate blanket 6of elastomeric polyurethane material having an additive to control theresistivity and having a hardness of between about 60 Shore A and about95 Shore A. The elastomeric polyurethane may be from about 0.060 toabout 0.25 inch in thickness, having sufficient resiliency to allow theroll to deform when brought into moving contact with the photoconductivedrum surface to provide an extended contact region in which the tonerparticles can be transferred between the contacting bodies. At thethinner range of preferred thickness the elastomeric polyurethane layeris relatively hard while at the thicker range of thickness it isrelatively soft. Intermediate blanket 6, which is comparable to layer 4in FIG. 1, should be capable of responding rapidly to the biasingpotential to impart electrically the charge potential on the core to theouter extremities of the roll surface. The blanket therefore should havea D.C. volume resistivity of between about 10⁷ and 5.0×10¹¹ ohm cm, andpreferably about 10⁹ to about 10¹⁰ ohm cm. Over the intermediate blanket6 is placed a relatively thin outer coating 8 which may be anelastomeric material such as a polyurethane having a D.C. volumeresistivity of between 10¹⁰ and 10¹⁵ ohm cm and which preferably has athickness of about 0.001 inch and a hardness of about 85-95 Shore Adurometer.

Polyurethane elastomers are typically produced by the reaction of apolyisocyanate and a polyether containing hydroxyl groups according tothe general reaction:

    R.sub.a NCO+R.sub.b OH→R.sub.a NHCOOR.sub.b

In the practice of the present invention the polyurethane elastomer ismade by the reaction of a polytetramethylene ether glycol forming thebase polymer chain which has the formula HO[(CH₂)₄ O]_(x) H wherein x isfrom about 8 to about 41 providing a molecular weight range of the orderof from 650 to 2,900. In a preferred embodiment, wherein the overallmechanical properties including resiliency, hardness, compression set aswell as toughness are relatively stable over a range of temperature andrelative humidity distributions normally encountered in an officeenvironment, x is between 39 and 41. Within this higher range ofmolecular weights of the glycol a relatively soft segment is provided inthe polyurethane elastomer enabling a high level of resiliency over arelatively broad temperature range. The preferred polytetramethyleneether glycols are those having molecular weights near the higher end ofthe stated range and in particular the polyether glycol Terathane™ 2900available from E. I. DuPont de Nemours, Inc. which has a molecularweight of about 2900.

The diisocyanate is selected from the group consisting of methylenediisocyanates, diphenylmethane diisocyanates, toluene diisocyanates,naphthalene diisocyanates and blends thereof and is used in amounts offrom about 20 to about 95 parts by weight per 100 parts by weight of theglycol. The functional NCO groups of the diiocyanate provide arelatively hard and rigid segment in the final polymer chain and actvery much like a filler to provide a tough but flexible structure thathas both hard and soft domains. Typical diisocyanates useful in thepractice of the present invention include 4,4' diphenylmethanediisocyanate, 2,4' diphenylmethane diisocyanate, 2,4-toluenediisocyanate, 2,6-toluene diisocyanate and naphthalene 1,5-diisocyanateas well as blends and mixtures thereof. A particularly preferred blendof diisocyanates is one containing 98 percent 4,4' diphenylmethanediisocyanate and 2 percent 2,4' diphenylmethane diisocyanate availableunder the designation isocyanate™ 125M from Dow Chemical Company,Midland, Mich.

In a preferred embodiment, with the preferred higher molecular weightglycols, the diisocyanate or blend thereof is present in an amount offrom about 22 to 26 parts by weight per 100 parts by weight of theglycol to provide the stoichiometric amount for the reaction to go tocompletion.

The optimization of the mechanical properties, and in particular, theachievement of high wear resistance with low compression set is attainedby the selection of the type and amounts of chain extenders and crosslinking agents. Both bifunctional chain extenders and trifunctionalcross linking agents are used in the practice of the present inventionin a weight ratio of from about 90% to 60%, to 10% to 40% ofbifunctional to trifunctional agents with the higher ratio being usedwith the lower molecular weight glycols. The bifunctional agents havebeen found to provide a higher toughness, provide more chain extensionin that they tend to linearly link the chain thereby providing a long,generally flexible glycol chain interrupted by the rigid isocyanateunits. The trifunctional crosslinkers tend to provide two ends offunctionality as well as a 90 degree oriented functional member whichcrosslinks to other chains prohibiting the chains to slide by each otherand thereby minimizing the compression set and tensile set properties aswell as the mobility of the additive through the elastomer. Accordingly,the appropriate bonds between hard and soft sites in the polyurethaneelastomer are obtained by selecting the bifunctional chain extenders andtrifunctional cross linking agents in the appropriate ratio. Typically,the bifunctional chain extenders take the formula HO(R₆)OH where R₆ is astraight or branched chain alkyl group having from 2 to 12 carbon atomsand the trifunctional cross linking agents are taken from the formula:R'--C--[(OH)_(a) (CH₂ OH)_(b) ] where R' is H, CH₃ or C₂ H₅, a is 0 or1, b is 2 or 3 and a+b=3. Typical bifunctional diols include ethyleneglycol, 1,4 butanediol, 1,3 butanediol 1,6 hexanediol and neopentylglycol and typical trifunctional triols include trimethylolpropane,trimethylolethane and glycerol. Particularly, preferred bifunctionalcross linking agents include 1,4 butanediol, 1,6 hexanediol and 1,3butanediol because they extend the polymer chain linearly yielding toughwear resistant materials. The particularly preferred trifunctional crosslinking agents include trimethylolpropane and trimethylolethane becausethey cross link the polymer chains at 90° and yield very set resistantnetworks. The bifunctional butanediol acts as a chain extender to extendthe chain in the linear way to provide linear soft sites therebyproviding the greatest toughness in the final elastomer. On the otherhand, the trifunctional trimethylolpropane provides the best compressionset performance because it is trifunctional and provides crosslinkexchange sites to tighten up the network, thereby providing acrosslinked, three-dimensional network. An amount of combined agents isused to provide a satisfactorily cross linked elastomer. Typically, thetotal amount of combined extenders and cross linking agents is fromabout 4 to about 18 parts per 100 parts of the polytetramethylene etherglycol depending on the molecular weight of the glycol with more agentsbeing used with lower molecular weight glycols. In the preferredembodiment with glycols having high molecular weight of the order 2900and smaller amounts of the diisocyanate of the order of about 22 to 26parts by weight per 100 parts of the glycol only about 4 to 6 parts byweight of extenders and cross linking agents is necessary because of thelong glycol chain length with limited number of functional groups.

The asymmetric ionic quarternary ammonium salt additive according to thepresent invention has the formula: ##STR3## where R₁, R₂, R₃, R₄ and R₅are C_(n) H_(2n+1) and 1≦n≦25. While R₁, R₂, R₃, R₄, and R₅ can be thesame it is preferred that they all not be the same but rather that theybe as different as possible to cause the greatest asymmetry andtherefore low additive mobility leading to the longest electrical life.Typical asymmetric ionic quarternary ammonium salts useful in thepractice of the present invention include, among others, Hexadecyl ethyldimethyl ammonium ethyl sulfate, (C₂₀ H₄₄ N.C₂ H₅ O₄ S);1-Octadecanaminium, N, N, N-trimethyl-,methyl sulfate (C₂₁ H₄₆ N.CH₃ O₄S); 1-Dodecanaminium, N, N, N-trimethyl-, methyl sulfate (C₁₅ H₃₄ N.CH₃O₄ S); 1-Heptadecanaminium, N, N, N-trimethyl-, methyl sulfate (C₂₀ H₄₄N.CH₃ O₄ S); 1-Tetradecanaminium, N, N, N-trimethyl-, methyl sulfate(C₁₇ H₃₈ N.CH₃ O₄ S); and 1-Decanaminium, N, N, N-trimethyl-, methylsulfate (C₁₃ H₃₀ N.CH₃ O₄ S). Hexadecyl ethyl dimethyl ammonium ethylsulfate is particularly preferred since it provides an outstandingimprovement in electrical life over prior art practices. The saltstypically are present in the cured elastomer in an amount sufficient toprovide a volume resistivity of from about 10⁷ to about 10¹¹ ohm cm.Typically, the quarternary ammonium salt is present in an amount of fromabout 0.5 parts to 8 parts by weight of the total composition: As theamount of conductive additive increases, the resistivity of theelastomer decreases. For the higher molecular weight materials theresistivity is influenced more pronouncely by the increasing addition ofconductive additive than for the lower molecular weight materialsbecause they have a greater cross link density with less chain rotationand flexibility. Furthermore, since the elastomer layer can be regardedas a dielectric material in a capacitor as the thickness of the layerincreases, the resistivity requirements decrease.

A catalyst is typically used to speed up the rate of reaction of thecross linking and extending mechanisms to provide the cured polyurethaneelastomers. Typical conventional catalysts include dibutyl tin dilaurateand stannous octoate in a 1% to 2% solution of the diol extender.

The polyurethane elastomer may be made according to any suitableprocedure. For example, all the reactive ingredients including thecatalyst may be added at one time or serially to a single reactor vesselto produce the polyurethane elastomer. However, this procedure resultsin a poorly controlled reaction in that there are two reactions takingplace simultaneously; one between the glycol and the diisocyanate andthe other between the reaction product of the first reaction and themixture of extenders and cross linking agents. Thus, formation of aprepolymer, chain extension and cross linking all occur at the sametime. Accordingly, it is preferred to prepare a prepolymer of at least aportion of the glycol with at least a portion of the diisocyanate toenable the reaction of the NCO groups of the isocyanate with the OHgroups of the glycol to form a long chain so that the NCO groups can'tsubsequently take up water and retain it in the final polyurethaneelastomer presenting an ultimate problem in that it results inunpredictable properties dependent on relative humidity. For the lowermolecular weight glycols, since the curing of the elastomer isexothermic, it is preferred that they only be prepared using theprepolymer method. The prepolymer method provides an initial lowmolecular weight polymeric diisocyanate and provides better control overthe polyurethane formation reaction and eliminates the formation ofmonomeric diisocyanate. Once the prepolymer, which is typically aviscous liquid, has been formed the mixture of chain extenders and crosslinking agents may be added together with the catalyst to form thepolyurethane elastomer. Alternatively, the reaction may be suspendedafter it's initiated by freezing the reactants at a temperature of theorder of 40° below zero Fahrenheit and the reaction completed at a laterdate by placing the frozen reactants, for example in an appropriatelyheated tool to make a part. Once all the reactants have been addedtogether and the polymerization reaction has been initiated the formingpolyurethane may be shaped according to any of the conventionaltechniques including injection molding, spin casting, flow coating,compression molding, mold casting, etc. Following shaping thepolyurethane elastomer may be cured at elevated temperature from about200 to 250 degrees Fahrenheit for approximately 1 to 2 hours followed bya postcure at the same temperature for up to 16 hours and apreconditioning at room temperature for about two weeks. While theasymmetric ionic quarternary ammonium salt may be added with the otherreactive ingredients to the reactor vessel, it is preferred to first addit to the mixture of the cross linking agents and chain extenders thenheating to dissolve the additive followed by filtering to removeimpurities, degassing at about 176° F. and then adding to theprepolymer. Typically, the additive is present in amount of about 0.5parts to 8 parts by weight per 100 parts of the total composition.

In fabricating a bias transfer member, typically a roll, a conductivesubstrate such as aluminum is first grit blasted and then cleaned withtrichloromethane. While not absolutely necessary, it is preferred toprime the aluminum substrate with a primer such as Chemlock 213, a blendof aromatic and aliphatic hydrocarbon solvents and fillers, mixed with athinner such as Chemlock 248, a blend of methyl ethyl ketone, xylene,propylene glycol, methyl ether acetate and dipropylene glycol methylether, both available from Lord Elastomer Products, Erie, Pa. in avolume ratio of about 1 to 10 of primer to thinner to provide a thincoating that does not effect the resistivity of the polyurethaneelastomer.

The higher molecular weight glycols form relatively soft elastomershaving a Shore A hardness of 60 durometer and in which it is possible toincorporate substantial quantities of the conductive additive to obtainelastomers having very low resistivities which may be desirable incertain applications. Furthermore, the higher molecular weight,polytetramethylene ethyl glycols are much more difficult to clean astransfer members of machine debris, including toner than the much lowermolecular weight glycols. For example, it takes more than four times asmuch air velocity (416 versus 96 feet/second) to blow debris off anelastomer surface made with a polytetramethylene ether glycol having amolecular weight of about 2900 than one having a molecular weight ofonly about 650. Accordingly, the best properties of both the highmolecular weight and low molecular weight glycols may be had byproviding a relatively thick (0.250 inches) layer of the high molecularweight material on the roll surface in which the resistivity can becontrolled at low levels and spray coated with a thin over coat of therelatively low molecular weight materials to a thickness of from aboutseven-tenths of a mil. to one and one-half mils. and nominally one mil.

The following specific examples illustrate more clearly the electricallyconductive polyurethane elastomers according to the present invention.In the Examples, all parts and percentages are by weight unlessotherwise stated.

EXAMPLE I

Sample rolls were fabricated from a polyether polyurethane material thatwas comprised of components A and B. Component A consisted of 100 partsby weight of Terathane™2900, 0.8 parts by weight of Catafor CA-100 whichis a hexadecyl ethyl dimethyl ammonium ethyl sulfate available fromAceto Chemical Company in Flushing, N.Y.; 5.8 parts by weight of amixture of 60% butanediol and 40% trimethylol propane and 4 to 8 dropsof dibutyl tin dilaurate catalyst in a one percent solution ofbutanediol to control the reaction rate. Component A was fabricated bypreheating the butanediol trimethylol propane mixture and the Catafor to80° C. and also preheating the Terathane 2900 to 80° C. Component B wascomprised of 24 parts by weight of methylene diisocyanate preheated to40° C. Components A and B were separately degassed for 30 minutes at 80°C. after which component B was poured into component A with mixing andthe mixture degassed for an additional minute. The material was thenpoured into a preheated mold and cured for 2 hours at 300° F. After thecuring, the mold was water quenched for 30 minutes and the molded partremoved from the mold. The roll was then permitted to dwell at roomtemperature from 3 to 24 hours and postcured at 300° F. for 16 hours.The fabricated rolls were then permitted to dwell for 16 days at roomtemperature to stabilize the crosslinking and were then ground using aSouth Bend grinder to a diameter of 1.5 inches. The fabricated rollswere then tested for resistivity and electrical life and compared to thebias transfer roll described in the above mentioned patents made from apolyester polyurethane with a tetraheptyl ammonium bromide conductiveadditive with the following results:

    ______________________________________                                        Property         Example I  Prior Art BTR                                     ______________________________________                                        Resistivity (ohm-cm)                                                                           2.62 × 10.sup.9                                                                    3 × 10.sup.9                                Electrical Life at 75 ua                                                                       900 hours  100 hours                                         ______________________________________                                    

The electrical life test was conducted on a fixture with the rollcycling against the photoreceptor with the electrical life beingdetermined by the increased resistivity of the roller as a function oftime. When the power supply voltage limit is reached the electrical lifeof the roll has failed. The resistivity was tested with conventionalequipment. The above results indicate that the roll prepared accordingto the present invention obtained the required resistivity but yieldedan electrical life improvement of 9 fold over the prior art.

The roll of Example I was tested in a Xerox 5090 duplicator forphotoreceptor and toner compatibility and transfer function anddetermined to have good transfer efficiency. It did not damage thephotoreceptor even at the higher hardness level and was compatible withvarious toners in initial testing.

EXAMPLE II

A bias transfer roll was prepared in the same manner as in Example I,except that it was finally ground to provide an outside diameter of1.499 inches and was overcoated with a thin layer of a polyurethaneelastomer obtained by the reaction of a lower molecular weightpolytetramethylene ethyl glycol DuPont Terathane 650 in the followingmanner: A spray mix was prepared by adding 92.3 grams of 4,4 diphenylmethane diisocyanate to 16.5 grams of the Terathane 650 as homogeneousliquids. Since the reaction is exothermic it takes place over a periodof 15 to 20 minutes while stirring, after which it is permitted to standfor 2 hours followed by degassing. This is mix A:

Separately, 4.3 grams of hexadecyl ethyl dimethyl ammonium ethylsulfate, Catafor CA-100 were added to 20.9 parts of butanediol andstirred to assure complete dispersion, after which the mixture wasplaced in an oven at 140° F. until the liquid became clear. To thissolution was added 83.5 grams of melted Terathane 650 which wasthoroughly mixed and degassed. This is mix B:

The spray formulation was prepared by adding 10 grams of methyl ethylketone and 10 grams of ethyl acetate to mix A and separately adding 10grams of methyl ethyl ketone and 10 grams of ethyl acetate to 30 gramsof mix B, followed by the addition of 0.4 grams of a fluorosurfactant,FC-430 available from 3M Company. The mix A portion was maintained at105° F. followed by adding mix A to mix B and vigorously blending for 30seconds to form the spray mix. The roll previously prepared was placedin a fixture and rotated at 45 rpm and the spray mix is sprayed onto therotating surface of the roll in 30 to 50 passes to provide an ultimatecoating of the lower molecular weight polyurethane of 0.001 inches. Thecoated roll was subjected to a postcure for 16 hours at 230° F. and isbelieved to have an electrical life equivalent to the roll of Example I.

Thus, according to the present invention an electrically conductivepolyurethane elastomer having a dramatically increased electrical lifeis provided. Further, this elastomer is environmentally stable tochanges in relative humidity and temperature, relatively inexpensive andcompatible with the multilayered electroconductive photoreceptorsdescribed in U.S. Pat. No. 4,265,990.

The patents and copending patent application referred to herein arehereby specifically, totally and completely incorporated herein byreference.

While the present invention has been described with reference tospecific embodiments described herein it will be apparent that manyalternatives, modifications and variations may be made by those skilledin the art. Accordingly, it is intended to embrace all such alternativesand modifications as may fall within the spirit and scope of theappended claims.

We claim:
 1. An electrically conductive polyurethane elastomer obtainedby the reaction of a polytetramethylene ether glycol having the formulaHO[(CH₂)₄ O]_(x) H where x is from 8 to 41 and from about 20 to about 95parts by weight per 100 parts by weight of said glycol of at least onediisocyanate, a sufficient amount of cross linking agents and chainextenders to provide a crosslinked elastomer and an amount of anasymmetric ionic quarternary ammonium salt having the formula: ##STR4##where R₁, R₂, R₃, R₄ and R₅ are C_(n) H_(2n+1) and 1≦n≦25 sufficient toprovide a D.C. volume resistivity of from about 10⁷ to about 10¹¹ ohmcm.
 2. The elastomer of claim 1 wherein said asymmetric ionicquarternary ammonium salt is selected from the group consisting ofHexadecyl ethyl dimethyl ammonium ethyl sulfate; 1-Octadecanaminium,N,N,N-trimethyl-,methyl sulfate; 1-Dodecanaminium, N,N,N-trimethyl-,methyl sulfate; 1-Heptadecanaminium, N,N,N,trimethyl-, methyl sulfate;1-Tetradecanaminium, N,N,N-trimethyl-, methyl sulfate; and1-Decanaminium, N,N,N-trimethyl-, methyl sulfate.
 3. The elastomer ofclaim 2 wherein said asymmetric ionic quarternary ammonium salt ishexadecyl ethyl dimethyl ammonium ethyl sulfate.
 4. The elastomer ofclaim 1 wherein said diisocyanate is selected from the group consistingof methylene diisocyanates, diphenylmethane diisocyanates, toluenediisocyanates, naphthalene diisocyanates and blends thereof.
 5. Theelastomer of claim 1 wherein said chain extender and cross linking agentcomprise a mixture of from about 90 to 60% by weight of a diol havingthe formula HO(R₆)OH where R₆ is a straight or branched chain alkylgroup having from 2 to 12 carbon atoms and from about 10 to 40% of atriol having the formula: R'--C--[(OH)_(a) (CH₂ OH)_(b) ] where R' is H,CH₃ or C₂ H₅, a is 0 or 1, b is 2 or 3 and a+b=3.
 6. The elastomer ofclaim 4 wherein said chain extenders and cross linking agent comprise amixture of from about 90 to 60% by weight of a diol having the formulaHO(R₆)OH where R₆ is a straight or branched chain alkyl group havingfrom 2 to 12 carbon atoms and from about 10 to 40% of a triol having theformula: R'--C--[(OH)_(a) (CH₂ OH)_(b) ] where R' is H, CH₃ or C₂ H₅, ais 0 or 1, b is 2 or 3 and a+b=3.
 7. The elastomer of claim 1 whereinsaid quarternary ammonium salt is present in an amount of from about 0.5parts to 8 parts by weight of the total composition.
 8. The polyurethaneof claim 1 wherein the elastomer is prepared by adding all the recitedreactive constituents and cross linking agents and extenders to areaction vessel at the same time.
 9. The polyurethane of claim 5 whereinthe elastomer is prepared by first forming a prepolymer of thepolytetramethylene ether glycol and the diisocyanate followed by theaddition of the cross linking agent and the extender to the prepolymer.10. The polyurethane of claim 1 wherein the total weight of crosslinking agents and extenders is from about 4 to about 18 parts per 100parts of polytetramethylene ether glycol.
 11. The polyurethane of claim5 wherein the elastomer is formed from a mixture of a chain extender anda cross linking agent comprising about 75% by weight of 1,4 butanedioland 25% by weight trimethylolpropane.
 12. The polyurethane of claim 5wherein the elastomer is formed from a mixture of a chain extender and across linking agent comprising about 60% by weight 1,4 butanediol and40% by weight trimethylolpropane.
 13. The polyurethane of claim 1wherein x is from 39 to
 41. 14. The polyurethane of claim 1 wherein saiddiisocyanate is present in an amount from about 22 to 26 parts by weightper 100 parts by weight of said glycol.
 15. The elastomer of claim 9wherein said quarternary ammonium salt is initially combined with thechain extenders and cross linking agents and subsequently added to theprepolymer.